1. Field of the Invention
This invention relates to a rapid process for mechanically dewatering and thermally drying aqueous slurries of calcium sulfate hemihydrate, and, in particular, alpha hemihydrate slurries.
In the slurry process for the preparation of alpha calcium sulfate hemihydrate, the three major steps are the autoclave calcination of gypsum to the hemihydrate under high pressure and temperature conditions; mechanically dewatering the aqueous hemihydrate slurry; and then thermally drying to produce the desired dried product. Ever since the inception of the slurry process, those skilled in the art have been cautioned to maintain the material from the autoclave through the drier at high temperature in order to avoid rehydration of the calcium sulfate hemihydrate. The temperature of the mass is normally maintained substantially in excess of 140.degree. F. (60.degree. C.) until the mass of calcined gypsum crystals is completely dried, in order to avoid problems caused by adhesiveness and cohesiveness of the hemihydrate. As the slurry is being converted to dried solids the hemihydrate solids become more sticky as additional water is removed.
Any substantial cooling of the hot mass causes the hemihydrate material to stick to the equipment, and deposits of hemihydrate rapidly build up on the equipment. Overlaid material results in further cooling and the material hydrates and forms a hard mass. Processing equipment must then be shut down and cleaned out before there is plugging and possible breakage of handling equipment and conveying lines. Clean surfaces can have deposits build up to the point of plugging in a matter of minutes, and then jack hammers must be used to clean the equipment.
2. Description of the Prior Art
Over the years, centrifugation has been frequently proposed as a means for dewatering the aqueous slurry, as mentioned in U.S. Pat. No. 1,941,188. That patent discloses that the calcined gypsum slurry is dewatered in a filter press, centrifuge or the like, while the temperature of the mass is maintained sufficiently high to prevent rehydration, and then, it is completely dried in a dryer such as a rotary dryer. It is readily apparent, however, that sufficient means to avoid stickiness of the dewatered damp material have not yet been developed for the centrifugation dewatering process. U.S. Pat. No. 3,377,020 discloses a process using a solid bowl, continuous centrifuge for the dewatering and applying heat to the solids discharge compartment at the exit of the centrifuge. In that process, the walls of the solids receiving compartment are lined and heated to a temperature of approximately 246.degree.-251.degree. F. (119.degree.-122.degree. C.) by a fluid or gas circulating in spaces between the inner and outer walls. Such heating is maintained to the drier through a lining in enclosed conveying equipment. U.S. Pat. No. 3,561,930 also discloses heating the casing of a solid bowl centrifuge, but to lower temperatures in this range and optionally assisting the heating by adding saturated steam near the liquid outlet to prevent evaporation and depositing of gypsum scale from the liquid effluent. It is believed that these processes have never been commercially successful.
Published German Federal Republic application No. 28 15 366, published Oct. 18, 1979, also proposes, in a process using a push centrifuge followed by a flow drying unit, that a walled space between an inner housing and an exterior housing of the discharge spout on the end of that centrifuge be heated. Further, it proposes diverting a portion of the hot air from the dryer through that space to heat the inner walls of the discharge spout to avoid sticking of the wet solids. Here also, the inner walls of the transition area from the discharge spout to the dryer appear to be heated by a continuation of the heated space between the inner and outer walls. It is believed this proposed solution has never been commercially practiced.
Thus, it is apparent that other means must be found in order to use successfully centrifugation for dewatering such slurries without the difficulties of deposit build-up and plugging by caked solids.